DIESEL ENGINESDOE-HDBK-1018/1-93Diesel Engine FundamentalsCompressionRatioand ClearanceVolumeClearance volumeis the volume remaining in the cylinder when the piston is at TDC.Because of the irregular shape of the combustion chamber (volume in the head) theclearance volume is calculated empirically by filling the chamber with a measured amountof fluid while the piston is at TDC. This volume is then added to the displacementvolume in the cylinder to obtain the cylinders total volume.An engine's compression ratio is determined by taking the volume of the cylinder withpiston at TDC (highest point of travel) and dividing the volume of the cylinder when thepiston is at BDC (lowest point of travel), as shown in Figure 15. This can be calculatedby using the following formula:Compression Ratio displacement volume clearance volumeclearance volumeFigure 15 Compression RatioHorsepowerPower is the amount of work done per unit time or the rate of doing work. For a dieselengine, power is rated in units of horsepower. Indicated horsepower is the powertransmitted to the pistonsby the gas in the cylinders and is mathematically calculated. ME-01Rev. 0Page 18